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概述
Layer图层是 Openlayers 中很重要的一个概念和部分，而无论是VectorLayer矢量图层还是VectorTileLayer瓦片图层都是继承BaseLayer实现的。本文主要介绍BaseLayer的核心部分以及实现。
源码剖析
BaseLayer类继承自BaseObject类，其实现如下：
class BaseLayer extends BaseObject {
constructor(options) {
super();
this.on;
this.once;
this.un;
this.background_ = options.background;
const properties = Object.assign({
}, options);
if (typeof options.properties === "object") {
delete properties.properties;
Object.assign(properties, options.properties);
}
properties[LayerProperty.OPACITY] =
options.opacity !== undefined ? options.opacity : 1;
assert(
typeof properties[LayerProperty.OPACITY] === "number",
"Layer opacity must be a number"
);
properties[LayerProperty.VISIBLE] =
options.visible !== undefined ? options.visible : true;
properties[LayerProperty.Z_INDEX] = options.zIndex;
properties[LayerProperty.MAX_RESOLUTION] =
options.maxResolution !== undefined ? options.maxResolution : Infinity;
properties[LayerProperty.MIN_RESOLUTION] =
options.minResolution !== undefined ? options.minResolution : 0